The present invention relates generally to devices, schemes and methods for network protection of electric power systems, and electric power systems comprising such devices and schemes. Network protection refers to measures in order to avoid or reduce a substantial disturbance in an electric power system. The invention relates in particular to such protection against poorly damped power oscillations in electric power systems.
Control and protections in electric power systems are of many different kinds. Single units are often provided with protection devices, which may detect any faults or if the unit is operated outside its limits. Such a protection device typically reduces the operation conditions or disconnects the unit, and is therefore only concerned about the local conditions.
In the present disclosure, xe2x80x9cpower systemxe2x80x9d and xe2x80x9cpower networkxe2x80x9d refers solely to electric power, even if not explicitly mentioned.
For system disturbances, where the whole or substantial parts of an electric power system are involved, system protection schemes are used, which detects the occurrence or an acute risk for occurrence of a major disturbance and provides measures to reduce the consequences. Such measures may e.g. be the disconnection of certain loads, the division of the power system network into smaller autonomously operating networks, etc. The situation, in which these system protection schemes are activated, are emergency or close to emergency situation, and the time for performing the necessary actions is very limited, typically in the order of a part of a second up to half a minute.
The present invention is aimed at indicating and protecting electric power systems against instability due to power system oscillations not sufficiently damped. An electric power system is in general terms considered stable if the oscillatory response of the power system during the transient period following a disturbance is damped and the system settles in a finite time to a new steady-state operating point. Power system stability depends on the existence of both the synchronizing and the damping torque. Lack of sufficient synchronizing torque results in instability through an aperiodic drift in rotor angle, while lack of sufficient damping torque results in oscillatory instability, i.e. oscillations of increasing amplitude. Rotor angle stability in electric power systems can be divided into transient (angle) and small-signal stability. Small-signal stability is the ability of an electric power system to maintain synchronism under small disturbances, where a disturbance is considered small if linearization of the system equations is permissible for the purpose of analysis. The present invention is concerned with damping of powers system oscillations by providing additional damping torque to maintain small-signal stability.
In many countries there is an on-going restructuring of the electric power industry. This restructuring includes deregulation, and in some cases privatization, of electric utilities. These changes in power markets around the world have led to substantially reduced investments in infrastructure, i.e. investments in hardware. A continuously increasing load level in combination with new power flow directions has led to that new operation conditions may appear for the system operators with respect to earlier operation conditions in well known electric power systems. New operation situations and fast production changes and power flow changes associated therewith increases the demands on the tools and facilities of the operator to have a continuous overview and control of the operation security and margins in the electric power system. The demands for models, measurement data and calculation programs will thereby increase.
One of the causes for the at present, in many places, increasing interest in stability issues is that a load growth without a corresponding increase in transmission capacity has resulted in that many power systems today are being operated closer to their limits. During the last decades, there has been an increase in generation capacity as well as in use of electricity in the industrial world. A problem is the power delivery infrastructure, which is becoming more stressed in the new high-traffic and more competitive electricity industry. The power grids have over the years also become more widely interconnected and cover larger geographical areas. The power grids built and extended during past decades were, in many cases, not planned for handling the large number of transactions taking place in today""s deregulated power markets. As a consequence, power system damping has in some cases been reduced, which has led to an increased risk of poorly damped oscillations. As a result, there is likely a substantially increased risk for larger scale power system failures (black-outs).
The dependence in modern society on a reliable power supply must not be underestimated. Furthermore, more and more customers are today more and more sensitive to disturbances in the electric power systems. The increased focus on power quality (PQ) issues includes both unwanted variations in the power supply in form of e.g. voltage sags and dips as well as disruptions in the supply of power. For this reason, some of the existing defense plans have to evolve from systems designed in the 1960s or 1970s to meet the requirements of the actual power systems today. It is further not possible from a design viewpoint to build a power system that can withstand all contingencies that may occur.
In case of serious faults, combination of faults or extreme load or unexpected production changes in the power system, there are network protections at a number of locations over the world, which try to avoid extensive network breakdowns and instead limit the consequences and facilitate the recover of the network. The area of system protection schemes (SPS) comprises a number of different types of systems, where the information carrying signals may be control signals as well as information that certain measurement values have exceeded or fallen below their limits.
The defense plans of today against serious disturbances are mainly adapted for transient angle phenomena in the power network. These types of system protection schemes are mainly concerned with load disconnection or islanding of the network.
Small-signal stability can be enhanced by use of Power System Stabilizers (PSS), which is a device with a basic function of adding damping to the generator oscillations by modulation of the generator excitation control signal. Additional damping torque is achieved by modulating the generator excitation to develop a component of the electric torque in phase with rotor speed variations. The speed deviation is therefore a logical signal to use for controlling the excitation of the generator by using auxiliary stabilizing signals. However, in practice both the generator and the exciter exhibit frequency dependent gain and phase characteristics.
Conventional PSS devices are local, using exclusively local measurement for decisions on how to control generator excitation to damp power system oscillations. Commonly used input signals to a PSS for stabilization of poorly damped power oscillations via excitation control include speed deviation, frequency, electric power and accelerating power. These input signals are processed for to find indications of power system oscillations, which manifest themselves as variations in power, currents, voltages etc. The analysis is thus based on an indirectly obtained indication of power oscillations and therefore only gives an indirect picture for the rotor position of the generator. Additionally, supplemental stabilizing signals to enhance the damping of power system oscillations may be taken from modulation of generator input power control, control (switching) of active power loads, changes in power system operating conditions, control of SVC, HVDC converters and FACTS devices, etc.
The objectives of excitation control design is to maximize the damping of both local and inter-area modes of power system oscillations, without comprising the stability of other modes, as well as enhance power system transient stability. The measures to be taken to improve the power system small-signal (oscillatory) stability are typically designed for a specific mode of oscillations. Unusual and previously not studied operating conditions could lead to that fixed parameter settings for PSS provide insufficient additional damping torque, and could in (very) unfavorable situations even aggravate the oscillations. The possibility of distinguishing between different modes of oscillations and determine appropriate countermeasures based on local measurements, can be limited in prior art PSS devices in unusual and unfavorable network situations, e.g. where cascaded events progressively have weakened the electric power system.
A French system protection scheme is described in the article xe2x80x9cMajor Incidents on the French Electric System: Potentiality and Curative Measurementxe2x80x9d by C. Counan et. al., IEEE Transactions on Power Systems, Vol. 8, No 3, August 1993, pp. 879-886. The system is built up in a hierarchic structure, where detection devices are scattered over the network according to a certain configuration. The detection devices are connected to a central analyzing unit, determining the risk for disturbances. The detection devices detect voltage beats by monitoring the variations of local voltage. In case of disturbances, the network is fragmented upon request of the central analyzing unit into isolated islands, having one or several detection devices. The scheme is mainly intended for transient angle disturbances. The measure to meet the disturbances, by load shedding or fragmenting of the power network is, however, quite crude and in most cases not the appropriate measure for handling oscillations, since the integrity of the power network is destroyed.
A network protection system in southern Sweden against voltage collapse has been designed jointly by Svenska Kraftnxc3xa4t, Vattenfall A B and Sydkraft A B and is described in xe2x80x9cSpecial Protection Scheme against Voltage Collapse in the South Part of the Swedish Gridxe2x80x9d, by B. Ingelsson et. al., CIGRÉ Paper 38-105, Paris, August 1996 or xe2x80x9cWide-Area Protection Against Voltage Collapsexe2x80x9d by B. Ingelsson et. al., IEEE Computer Applications in Power, Vol. 10, No 4, October 1997, pp. 30-35. The objective of the network protection system is to avoid a voltage collapse after a severe fault in a stressed operation situation. The system can be used to increase the power transfer limits from the northern part of Sweden or to increase the system security or a mixture of both. This system is, however, not suitable for detecting and counteracting power oscillations.
A general problem with existing system protection schemes is that the measured quantities often are insufficient to efficiently detect the power system oscillation disturbances or the mode of oscillation.
Equipment for measurement of complex ac quantities (amplitude and phase, phasor measurements) and systems for evaluating the risk for instability, based on local measurement quantities have recently been made available. Measurement and collection of time stamped complex quantities, phasor quantities, with respect to current and voltage can be performed by means of a Phasor Measurement Unit (PMU). These units comprise a very accurate time reference, achievable e.g. by using the Global Positioning Satellite (GPS) system. Such systems are installed e.g. in northwestern USA to record conditions of power systems and are used for a post-evaluation of an emergency situation. See e.g. xe2x80x9cWide Area Measurements of Power System Dynamicsxe2x80x94The North American WAMS Project and its Applicability to the Nordic Countriesxe2x80x9d Elforsk Report 99:50, O. Samuelsson, Technical University of Lund, January 2000.
However, such data has not been considered to be involved in any system protection scheme used for detecting and counteracting power oscillations.
An object of the present invention is thus to provide a system protection scheme for improved detection and damping of power system oscillations. A further object is to make use of the information contained in time stamped quantities and quantities derived therefrom as a base for protection decisions. Another object is to obtain information about the oscillatory relation between different positions in the power system. Another object of the present invention is to provide a system protection scheme, which has a fast response to indications of oscillatory disturbances. The response time should preferably be independent of external factors.
Schemes, devices, systems and methods according to the enclosed claims achieve the above objects. In general words, power system oscillations can according to the present invention be detected by evaluating the angle difference between phasor quantities measured at different, at least two, locations in the electric power network. The measurements of voltage, current or power could be taken from different areas in a power system or over a single line, cable or interconnection. Actions against detected power system oscillations are taken in at least one location.
Therefore, a number, at least two, of system protection terminals are introduced at suitable locations in the electric power system. The system protection terminals are interconnected by a communication system, using a substantially dedicated communication resource.
A number, at least two, of the system protection terminals are equipped to collect measurement signals associated with the characteristic of the power system at that particular location. The measurements comprise time stamped voltage and/or current values and/or complex ac quantities. The signals are processed and data related to the measurements are spread on the dedicated communication resource to the other system protection terminals. This processed data preferably comprises the rotor angle of generators in that particular location.
The system is equipped with evaluation means to evaluate the oscillatory condition of the power network and if necessary provide control signals to power system units to counteract poorly damped power oscillations. The evaluation is based on selected parts of the data available on the communication resource, locally available data and/or externally entered data. Preferably, the evaluation is based on differences in rotor angles between different locations in the power system.
A number of the system protection terminals are provided with control signal providing means, which provide power system units with suitable control signals for counteracting power oscillations found by the evaluation of measurements. Preferably, these system protection terminals also comprise the evaluation means.
Preferably, the system protection terminals have local means for storing data. The data comprises the near history of system information as well as older measurements. The storing means are used e.g. to provide (latest available) information about conditions in surrounding areas during autonomous operation conditions, i.e. in situations when the communication fails. The stored data is also preferably used to follow up stressed situations in a post-analysis, e.g. during restoration after a full or partial power system black-out. Preferably, the storing means are searchable databases.
The substantially dedicated communication resource connecting the system protection terminals is designed with a high capacity. For protection against damping of system wide power oscillations, the requirements on the communication time are of the order of fractions of a second.
For systems with a multitude of system protection terminals, each system protection terminal has preferably access to at least two links of the communication system, providing a first degree of redundancy concerning communication failures. Each system protection terminal comprises a processor and suitable means for the communication. Preferably, a local database is provided for each terminal.
By having access to time stamped information from more than one location within the power network, any changes in the rotor angle difference between these locations may easily be detected. Changes in rotor angle differences indicate directly power oscillations including the locations of the measurements. Furthermore, the frequency, damping and amplitude of the oscillation are easily monitored. The direct measurement of variations in node angles proposed in the present invention thus provides a better picture of the modes of power oscillations of interest, i.e. the synchronism between generators. Suitable counteracting measures may then be selected, i.e. an adaptive Power System Stabilizer (PSS) is accomplished.
The present invention also allows for adaptive settings of parameters of PSS and other equipment that could enhance damping of power system oscillations. In such cases parameters have to be set according to the mode of oscillation of interest, which requires an exchange of control information between the terminals, to enhance the overall power system stability.
Further advantages and examples are understood from the following detailed description.